Generally conventional type of trip meters are of the mechanical type. In such trip meters a mechanical counter including a numeral display device is driven via reduction gears by a drive shaft operatively connected to the transmission or other mechanism of the vehicle. A predetermined number of the rotations of the drive shaft corresponds to a predetermined unit distance so that the numerals indicative of the distance travelled by the vehicle increases in proportion to the number of the rotations of the drive shaft.
However, the distance derived from the conventional trip meter is apt to be erroneous since the substantial diameter of wheels of the vehicle is apt to vary in accordance with various conditions of the vehicle, such as the air pressure of each wheels and the weight of the load of the vehicle which is very much influenced by the number of passengers. Moreover, when wheels of the vehicle used for a relatively long period of time, the diameter of each of the wheels becomes smaller due to wear of the outer surface thereof. When the substantial diameter of each of wheels is different from a predetermined value, a predetermined number of rotations of the wheels which should correspond to a predetermined number of rotations of the drive shaft does not correctly correspond to a predetermined unit distance.
Therefore, in order to ascertain an accurate distance actually travelled, the vehicle driver had to modify the measured distance by an error factor. For obtaining the error factor the vehicle driver is supposed to measure the distance of a predetermined course the distance of which is already known, by the trip meter of the vehicle. Unless the error is zero, the vehicle driver has to divide the predetermined distance of the course by the measured distance to obtain the error factor and to multiply the error factor to the measured distance to correct the erroneously measured distance. The above mentioned calculations, i.e. the devision and multiplication are usually troublesome and then the vehicle driver had to put up with the erroneous distances.